The present invention relates to a method of preparing borazine modified polycarbosilane polymers. The method comprises reacting a polycarbosilane polymer with a material comprising at least one borazine ring for a time sufficient to produce the desired polymer. This invention also relates to the novel polymers produced by the above method and the ceramics derived from pyrolysis of the novel polymers to greater than about 800.degree. C.
A variety of polycarbosilane oligomers, cyclics, resins and linear polymers are known in the art. Generally, such polymers are characterized as having backbones with alternating silicon and carbon atoms. Polycarbosilanes are useful as precursors to silicon carbide-related ceramics. Metallic compounds have been incorporated into prior art polycarbosilanes to enhance the stability of the polymers and the resulting ceramic fibers. For example, Yajima et al. taught that improved thermal stability of polycarbosilane fibers can be achieved by incorporation of compounds including a titanoalkoxide (U.S. Pat. No. 4,342,712, issued Aug. 3, 1982) and zirconoalkoxide (U.S. Pat. No. 4,515,742, issued May 7, 1985). Similarly, Yajima et al., in U.S. Pat. No. 4,220,660, issued Sep. 2, 1980, described a process for producing polycarbosilanes partly containing siloxane bonds wherein polyborosiloxane containing phenyl groups in at least a part of the side chains of Si is reacted with a polysilane. However, Yajima et al. noted that most of the boron is dissociated and is released out of the reaction system. Moreover, while Yajima et al. formed polycarbosilane products containing a small amount of boron which had improved physical properties, they did not teach a simple derivatization of preformed polycarbosilane as described in the present invention.
Takamizawa et al., in U.S. Pat. No. 4,604,367, issued Aug. 5, 1986, described a process wherein an organoborosilicon polymer comprising the elements of silicon, carbon, boron and nitrogen is prepared from the reaction a borazine with a polydimethylsilane. However, Takamizawa et al. failed to teach that a borazine modified polycarbosilane could be made from a preformed polycarbosilane.
Seyferth et al. , in "Silicon Ceramics with a Dash of Boron" (Front. Organosilicon Chem. , Proc. Int. Symp. Organosilicon Chem., 9th, Meeting Date 1990, 15-27), described the preparation of [B.sub.10 H.sub.10.diamine].sub.x polymers achieved by reacting decaborane and dimethylbis(dimethylamino)-silane. While Seyferth taught that the simple chemistry described therein also can be applied to modifying mixed systems of silanes and siloxanes, the present article failed to include preformed polycarbosilanes.
The present invention describes a method of forming a borazine modified polycarbosilane by reacting a borazine with a polycarbosilane. The present invention also describes novel borazine modified polycarbosilane polymers and the ceramics derived from the pyrolysis of these polymers.